While he was working as a military architect, repairing and modernizing castles for Cesare Borgia, Leonardo da Vinci sketched in his notebook a design for a radically new kind of fortress.
In Leonardo's day cannon were already powerful enough to demand changes in the design of forts, but by and large the new castles of the day still looked much like medieval ones.
This is Manzanares el Real, a Spanish fortress constructed beginning in 1475. The rounded outline was intended to make the walls less vulnerable to cannon fire, and there are cannon ports, but otherwise it looks quite traditional.
Extrapolating from the changes taking place around him, Leonardo looked into a future in which cannons were much more deadly and a fortress would have to become something quite different.
Curators at the National Museum of Science and Technology in Milan built a model based on Leonardo's sketches, which you can see above.
Look at this model from the side, and you see something from World War I.
Sometimes the hype about Leonardo's genius gets annoying, but here you can see him really reaching into the future and imagining something that no one had ever seen before.